Numerous variations of window elements for insertion in fuselage airframes of aircraft are known from reference in the field. Some of these window elements have a forged window frame made of aluminium which forms the actual supporting structure of the window element. A window frame with a T-shaped cross-sectional geometry is known; however, such a window is known to bulge or deform as the pressure difference changes during ascent to cruising altitude. This deformation increases drag and is aerodynamically disadvantageous.
A conventional window frame has a circumferential fixing flange with a circumferential recess. The window frame is firmly joined to the inside of the fuselage airframe in the area of the recess in an edge area of the window aperture. The joining can be made, for example, by riveting, gluing, welding or the like. A mechanical reinforcement of the window aperture inside the fuselage airframe can be achieved.
The window frame further fixes a pressing frame by means of a plurality of fixing elements, which are distributed preferably uniformly over the circumference of the window frame, for example, in the form of eye bolts. The eye bolts are fixed with their fixing eyes on a circumferential cross-piece or web of the window frame, and are arranged approximately perpendicularly to the fixing flange, are riveted, for example. The eye bolts each have threaded sections for receiving screwed nuts at their ends pointing away from the fixing eyes. The threaded sections of the eye bolts are guidable through corresponding openings in the pressing frame.
In addition, the window frame has an inwardly directed, conical bearing surface for positioning a window package formed of an inner pane and an outer pane. The inner pane and the outer pane are in this case bordered by a circumferential window seal, which is pressed onto the conical bearing surface by means of the pressing frame. The window package with the window seal is positioned on the bearing surface of the window frame from the interior of the fuselage airframe and is centred and fixed by means of the pressing frame. By tightening the screw nuts on the threaded sections of the eye bolts, the pressing frame, the window package and the window frame are firmly clamped or biased with respect to one another.
In such window elements, the window frame has many functions. First, the window frame reinforces the window aperture in the fuselage airframe. In addition, the window frame ensures the pressure-tight abutment and centering of the window package and the window seal. Furthermore, the window package is secured against falling out and/or pushing out. Finally, the window frame forms an externally visible edge which ensure the most favorable possible aerodynamic transitions from the outer skin of the fuselage airframe to the outer window of the window package.
The window elements comprising window frames, that are currently used, have very complex constructions as a result of the numerous requirements described hereinbefore. As a result of the high mechanical stressing, these window frames must be forged from aluminium and then machine processed, where the precision requirements are very high. As a consequence of the required expensive construction of the window frames, the plurality of additional connecting elements and the supporting frame, the weight of the known window elements is very high.
In addition, assembly of the described window elements is expensive. First, it is necessary to position the window frame exactly in the window aperture and then rivet to the fuselage airframe in the edge area of the window aperture. Then, up to six eye bolts must be individually riveted to the window frame. The window package is then inserted in the window frame where a second person must check the correct fit of the window package from outside and correct if necessary. The pressing frame is then threaded onto the eye bolts and screwed tightly to the window frame by means of screw nuts. In this case, the pressing frame presses elastically onto the window package and holds this in the pre-defined position inside the window frame.
In addition, aerodynamic adverse effects are frequently produced by using the currently used window elements in fuselage airframes of aircraft. These effects occur because is approximately flush in order to ensure that the outer pane of the window package with the outer skin of the fuselage airframe, the fixing flange of the window frame of the described window elements has a circumferential recess with a thickness approximately corresponding to the material thickness of the sheet metal usually used to form the outer skin.
However, the outer skin of aircraft fuselage airframes frequently has different material thicknesses in different sections of the fuselage. Thus, the outer panes of the window elements used are not always completely flush with the outer skin in all sections of the aircraft fuselage. Furthermore, the pressure difference at high flying altitudes produces a buckling or bulging of the outer pane which can be up to 4 millimeters over the outer strake. These described aerodynamic effects results in perturbations of the flow around the fuselage airframe which can considerably impair the aerodynamic quality of the outer skin of the fuselage airframe of an aircraft, especially if a large number of window elements is used in the fuselage airframe.
As a result of previously indicated disadvantages of the of window elements, often used currently these are only suitable to a limited extent for use in aircraft having large dimensions and a large number of window elements resulting therefrom.